The present invention relates to a process for preparing an improved diaphragm for use in chlor-alkali electrolytic cells. The diaphragm of this invention has improved mechanical properties which result in superior electrical performance and increased energy savings.
The chlor-alkali industry currently employs a large number of electrolytic diaphragm cells for the commercial production of chlorine and caustic soda. These electrolytic cells have an anode contained in an anolyte chamber and a cathode contained in a catholyte chamber separated by a porous diaphragm. The diaphragm is generally formed by depositing a slurry of asbestos fibers directly onto the foraminous cathode. The cells contain brine which is electrolyzed to produce chlorine gas in the anolyte chamber and sodium hydroxide (caustic) in the catholyte chamber.
Technical advances in this field have generated various improvements in component service life and cell operating efficiency or energy savings. These technical developments include dimensionally stable anodes, polymer reinforced diaphragms, activated cathodes, and decreased anode/cathode gaps. The improved electrodes have lower overvoltages and increased service life, while the polymer reinforced asbestos diaphragm has reduced swelling which enables the anode/cathode gap to be significantly decreased.
Present technology for preparing reinforced asbestos diaphragms requires the use of various polymeric reinforcing agents which are added to a slurry of asbestos fibers prior to deposition onto a cathode. The polymers used in this application must be resistant to attack and degradation by the electrolytic solution and cell products. Typical polymers include the fluorocarbon polymers such as polytetrafluoroethylene and polychlorotrifluoroethylene.
After being deposited onto the cathode, the diaphragm/cathode structure is heated to the fusion point of the polymer and subsequently cooled to room temperature. The deposition of the slurry is effected by means of a vacuum. Polymer-reinforced diaphragms of this type are disclosed in U.S. Pat. No. 4,410,411, issued Oct. 18, 1983 to Fenn et al., U.S. Pat. No. 4,142,951, issued Mar. 6, 1979 to Beaver et al., and Canadian Pat. No. 1,027,898 to Rucker. U.S. Pat. No. 4,142,951 also discloses that various surfactants, wetting agents, dispersing agents, modifiers or other processing aids can be added to the asbestos slurry in order to improve the dispersion of the asbestos fibers and fluorocarbon polymer and to impart increased porosity to the diaphragm. Titanium dioxide is listed in this patent as such a processing aid.
Although the polymer-reinforced diaphragms of the prior art do possess improved mechanical stability as compared to unmodified asbestos diaphragms, there are still opportunities for further technical improvements. For example, polymer-reinforced diaphragms prepared from polymers which are less resistant to the cell environment swell after a few days exposure to the cell environment, and the polymer itself tends to be degraded over a period of time, losing its capacity to effectively bond the fibers. Alternatively, when using more environmentally resistant polymers, the diaphragm-deposited cathode must be heated to the fusion temperature of the polymer which is typically in the range of about 350.degree. C. In addition to requiring more expensive heating furnaces, the use of such high temperature conditions can accelerate the mechanical degradation of the cathode and diaphragm.
An attempt to overcome the shortcomings of polymer-modified asbestos diaphragms is disclosed in U.S. Pat. No. 4,180,449, issued Dec. 25, 1979, to Heikel. This patent utilizes an organic titanate, such as tetraisopropyl titanate, which is dissolved in a solvent capable of wetting the asbestos fibers, such as anhydrous isopropanol. The titanate solution is used to impregnate a diaphragm which has been previously deposited onto a cathode member by vacuum deposition. The diaphragm is dried prior to treatment with the titanate solution to prevent hydrolysis of the titanate compound. The titanate contained in the diaphragm must then be hydrolyzed prior to pyrolysis. Hydrolysis is carried out in the presence of a hydrolyzing agent, such as water vapor, while pyrolysis occurs at temperatures of about 400.degree. C. The diaphragm produced according to this process is stated to be more durable and stable than unmodified diaphragms. However, this multistep process is both cumbersome and expensive to run commercially, and in addition it still requires very high heating temperatures.
It is therefore a principle objective of the present invention to provide an improved process for preparing an electrolytic chlor-alkali cell diaphragm which has superior physical and electrical properties in comparison to diaphragms disclosed in the prior art and those currently in commercial use.